1. Field of the Invention
The present invention relates to a sintered alloy composition and a manufacturing method thereof, and more particularly to a sintered iron alloy composition having excellent machinability and abrasion resistance under high bearing pressure, preferably to be used in making slide members for valve operating systems of internal combustion engines.
2. Description of the Prior Art
Conventionally, machine parts such as slide members of valve operating systems for internal combustion engines have been manufactured by using ingot material. However, in accordance with recent trends requiring high-performance engines, various sintered iron alloys have been developed and put to practical use. Such alloys have been provided for the purpose of improving abrasion resistance and machinability and lowering manufacturing costs of machine parts.
One example of sintered iron alloys with improved properties has been disclosed in Japanese Laid Open Patent Publication (Kohkai) No. S51-119419 filed on Apr. 11, 1975, by Hitachi Powdered Metals Co., Ltd. et al. This sintered iron alloy is composed of a pearlite iron base to which copper and tin are added in order to reinforce the iron base, which is characterized in that an iron-carbon-phosphorus ternary alloy is precipitated in the pearlite iron base, with free graphite being dispersed in the iron base. This sintered iron alloy has been employed as a material for valve guides for automobile engines.
Examples of other systemic sintered iron alloys have been suggested in Japanese Laid Open Patent Publication (Kohkai) Nos. S51-41619, S58-177435 and S61-243156. Disclosed in these documents are sintered iron alloys having an iron base to which components such as nickel, chromium, molybdenum, manganese, tungsten, vanadium, copper and the like are added for reinforcement of the iron base. Moreover, in these alloys, hard metal particles are dispersed in the iron base, as is needed, and a solid lubricant, such as a sulfide, lead or graphite, is dispersed in the iron base for the purpose of improving the abrasion resistance of the sintered iron alloy.
As described above, it is common to add additional alloy components to the alloy base when the iron base is to be reinforced so as to improve abrasion resistance under high loads in order to meet the latest requirement for high-performance internal combustion engines. Although this base-reinforcing method improves the abrasion resistance of the sintered iron alloy, it leads, for the most part, to lowered machinability of the alloy material. Accordingly, if such an alloy is employed as a material for slide members of valve operating systems for engines, difficulties arise in relation to the assembling process of internal combustion engines and the like. Namely, in the assembling process of the engine, the slide members, to which suction valves or exhaust valves are assembled, are provided on the cylinder head before machining is carried out, with the machining step being synchronized with other steps of the engine assembly. Consequently, low machinability of the slide members leads to increased machining time and further necessitates the use of several machining tools, thus hampering the total flow of the engine assembly process.